IPS LCD device having a wider viewing angle

ABSTRACT

An in-plane-switching-mode (IPS) LCD device includes a TFT substrate and a CF substrate sandwiching therebetween an LC layer, and a pair of polarizing films sandwiching therebetween the substrates and the LC layer. Each polarizing film has a polarization layer and a protective layer An optical compensation layer having a birefringence is disposed between the light-emitting-side polarizing film and the CF substrate. The optical compensation layer has an in-plane retardation of N 1  satisfying the following relationship:
 
83.050−0.810× D   1   ≦N   1 ≦−228.090−0.74 D   1  
 
in the range of 0&lt;D 1 ≦80 μm, wherein D 1  is the thickness of the protective layer of the light-incident-side polarizing film.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to an in-plane-switching-mode liquidcrystal display (IPS LCD) device and, more particularly, to theimprovement of an IPS LCD device to achieve a wider viewing angle.

(b) Description of the Related Art

An IPS LCD device has the advantage of a wide viewing angle due to theconfiguration wherein the twisting direction of the liquid crystal (LC)is parallel to the surface of the substrates. The IPS LCD devicegenerally includes an LC layer, a pair of glass substrates sandwichingtherebetween the LC layer, and a pair of polarizing films sandwichingtherebetween the glass substrates and the LC layer together. In the IPSLCD device, the LC layer is applied with a lateral electric field, whichis parallel to the substrates, to control the direction of the LCmolecules for image display.

It is known in the IPS LCD device that a chromaticity shift occurs,especially if the LCD device is observed in an angular direction of 45degrees away from the polarization direction of the polarizing films.The chromaticity shift degrades the image quality of the LCD device.Patent Publication JP-A-10(1998)-307291 and -2001-242462, for example,describe techniques for suppressing the chromaticity shift in the IPSLCD device.

FIG. 19 shows a portion of the IPS LCD device described inJP-A-10-307291 in a sectional view, wherein a TFT substrate 211 and anassociated polarizing film 219 on the TFT-substrate side are depicted,The polarizing film 219 includes a PVA (polyvinyl-alcohol) polarizationlayer 219C, a pair of triacetyl-cellulose (TAC) protective layers 219Band 219C sandwiching therebetween the PVA polarization layer 219D, andan optical compensation layer 219A disposed between the TAC protectivelayer 219B and the TFT substrate 211.

The TAC layer 219B has a negative retardation, whereas the opticalcompensation layer 219A has a positive retardation having a valueequivalent to the absolute value of the negative retardation of the TAClayer 219B. It is recited in this publication that the opticalcompensation layer 219A compensates the retardation generated in the TAClayer 219B, thereby reducing the polarization component caused by theretardation in the TAC layer 219B to suppress the chromaticity shift.

In the structure described in the JP-A-10-307291, however, thepolarization of the light incident onto the light-emitting-sidepolarizing film changes due to the retardation of the LC layer and thelight dispersion caused by the color filters. Thus, the suppression ofthe chromaticity shift by reducing the leakage light in the aboveconfiguration is insufficient.

FIG. 20 schematically shows the IPS LCD device described inJP-A-2001-242462 in an exploded perspective view thereof. The LCD deviceincludes two-axial retardation films 302 and 303 each disposed betweenthe LC layer 301 and a corresponding one of the pair of polarizing films304 and 305. The retardations of the retardation films 302 and 303 aresubstantially equal to the retardation of the LC layer 301. It isrecited in this publication that the retardation films 302 and 303should be preferably disposed so that the angles between thepolarization axis of the light-incident-side polarizing film 304 and theslow axes of the retardation films 302 and 303 assumes zero to 30degrees.

In JP-A-2001-242462, it is recited that a reversed transmittancephenomenon is suppressed by the configuration. The reversedtransmittance is such that the direction of the change in thetransmittance of the LC layer is reversed to the direction of the changein the driving voltage. In general, it is known in the IPS LCD devicethat the reversed transmittance phenomenon is observed in the wavelengthrange of green or blue when the driving voltage is changed for the LClayer in the range of intermediate gray scale levels. It is also recitedin the publication that the chromaticity shift caused by the change ofthe viewing angle is suppressed by the configuration.

In the LCD device described in JP-A-2001-242462, however, it is notconsidered there is leakage light caused by the deviation between thepolarization axis of the polarizing film 304 and the slow axes of theretardation films 302 and 303, as well as the leakage light caused bythe deviation between the polarization axis of the polarizing film 304and the optical axis of the LC layer 301. In general, the polarizationaxis of the polarizing film 304 deviates from the slow axes of theretardation films 302 and 303 by an angle of zero to 30 degrees. Thisdeviation generates leakage light upon display of black to degrade thecontrast ratio and thus degrade the image quality of the LCD device.

In view of the above problems in the conventional techniques, it is anobject of the present invention to provide an IPS LCD device, which iscapable of reducing the leakage light leaking upon display of blackcolor as observed in the slanted viewing direction to thereby improvethe contrast ratio of the IPS LCD device, and capable of suppressing thechromaticity shift between the normal viewing angle and a slantedviewing angle to thereby improve the image quality of the LCD device.

The present invention provides, in a first aspect thereof, an IPS LCDdevice including: a liquid crystal (LC) layer having a first opticalaxis; first and second substrates sandwiching therebetween the LC layer,the first and second substrates being disposed on a light-emitting sideand a light-incident side, respectively, of the LCD device; first andsecond polarizing films sandwiching therebetween the first and secondsubstrates and the LC layer, the first and second polarizing filmshaving polarization axes extending normal to each other and beingdisposed on the light-emitting side and the light-incident side,respectively, of the LCD device, the second polarizing film including afirst protective layer, a polarization layer and a second protectivelayer, which are disposed consecutively as viewed from thelight-incident side, each of the first and second protective layershaving a retardation depending on a thickness of the each of the firstand second protective layer; and an optical compensation layer disposedbetween the first polarizing film and the second polarizing film,wherein: the optical compensation layer has a birefringence satisfyingthe relationship (ns−nz)/(ns−nf)<≦0.5, where ns, nf and nz are arefractive index along an in-plane slow axis, a refractive index alongan in-plane fast axis, and a refractive index along a thicknessdirection, respectively, of the optical compensation layer; an anglebetween the in-plane slow axis of the optical compensation layer and thefirst optical axis is within ±2 degrees; and the optical compensationlayer has an in-plane retardation N₁ (nm), the second optical protectivelayer has a thickness D₁ (μm) satisfying the following relationship:83.050−0.810×D ₁ ≦N ₁≦228.090−0.74×D ₁,in a range of 0<D₁≦80.

The present invention also provides, in a second aspect thereof, an IPSLCD device including; a liquid crystal (LC) layer having a first opticalaxis; first and second substrates sandwiching therebetween the LC layer,the first and second substrates being disposed on a light-emitting sideand a light-incident side, respectively, of the LCD device; first andsecond polarizing films sandwiching therebetween the first and secondsubstrates and the LC layer, the first and second polarizing filmshaving polarization axes extending normal to each other and beingdisposed on the light-emitting side and the light-incident side,respectively, of the LCD device; and first and second opticalcompensation layers disposed between the first polarizing film and thefirst substrate and between the second polarizing film and the secondsubstrate, respectively, wherein: each of the first and second opticalcompensation layers has a birefringence satisfying the relationship(ns−nz)/(ns−nf)≦0.5, where ns, nf and nz are a refractive index along anin-plane slow axis, a refractive index along an in-plane fast axis, anda refractive index along a thickness direction, respectively, of theeach of the first and second optical compensation layers; an anglebetween the in-plane slow axis of the first optical compensation layerand the first optical axis is within ±2 degrees, and an angle betweenthe in-plane slow axis of the second optical compensation layer and thefirst optical axis is within 90±2 degrees; and the first and secondoptical compensation layers have in-plane retardations N₁ (nm) and N₂(nm), respectively, satisfying the following relationship:29.87+1.79N ₂−0.048N ₂ ²+0.001N ₂ ³ ≦N ₁≦187.22−1.66N ₂+0.0475N ₂²−0.0009N ₂ ³in a range of 0.6<N₂≦46.

The present invention also provides, in a third aspect thereof, an IPSLCD device including: a liquid crystal (LC) layer having a first opticalaxis; first and second substrates sandwiching therebetween the LC layer,the first and second substrates being disposed on a light-emitting sideand a light-incident side, respectively, of the LCD device; first andsecond polarizing films sandwiching therebetween the first and secondsubstrates and the LC layer, the first and second polarizing filmshaving polarization axes extending normal to each other and beingdisposed on the light-emitting side and the light-incident side,respectively, of the LCD device; and first and second opticalcompensation layers disposed between the first polarizing film and thefirst substrate and between the second polarizing film and the secondsubstrate, respectively, wherein: each of the first and second opticalcompensation layers has a birefringence satisfying the relationship(ns−nz)/(ns−nf)≦0.5, where ns, nf and nz are a refractive index along anin-plane slow axis, a refractive index along an in-plane fast axis, anda refractive index along a thickness direction, respectively, of theeach of the first and second optical compensation layers; an anglebetween the in-plane slow axis of each of the first and second opticalcompensation layers and the first optical axis is within ±2 degrees; andthe first and second optical compensation layers have in-planeretardations N₁ (nm) and N₂ (nm), respectively, satisfying the followingrelationship:162.560−8.874N ₂+2.258N ₂ ²−0.291N ₂ ³+0.0165N ₂ ⁴−0.000346N ₂ ⁵ ≦N₁≦142.465+2.546N ₂−0.017N ₂ ²in a range of 0.6≦N₂≦22;73.04−0.977N ₂+0.0220N ₂ ² ≦N ₁≦142.465+2.546N _(2−0.017)N₂ ²in a range of 22<N₂≦62; and73.04−0.977N₂+0.00220N ₂ ² ≦N ₁≦1205.596−41.304N ₂+0.586N₂ ²−0.0028 N ₂³in a range of 62<N₂≦92:

The present invention also provides, in a fourth aspect thereof, an IPSLCD device including: a liquid crystal (LC) layer having a first opticalaxis; first and second substrates sandwiching therebetween the LC layer,the first and second substrates being disposed on a light-emitting sideand a light-incident side, respectively, of the LCD device; first andsecond polarizing films sandwiching therebetween the first and secondsubstrates and the LC layer, the first and second polarizing filmshaving polarization axes extending normal to each other and beingdisposed on the light-emitting side and the light-incident side,respectively, of the LCD device; and first and second opticalcompensation layers disposed between the first polarizing film and thefirst substrate and between the second polarizing film and the secondsubstrate, respectively, wherein: the first optical compensation layerhas a birefringence satisfying the relationship (ns₁−nz₁)/(ns₁−nf₁)≦0.5,where ns₁, nf₁, nz₁ are a refractive index along an in-plane slow axis,a refractive index along an in-plane fast axis, and a refractive indexalong a thickness direction, respectively, of the first opticalcompensation layer; the second optical compensation layer has abirefringence satisfying the relationship (ns₂−nz₂)/(ns₂−nf₂)≦−2, wherens₂, nf₂, nz₂ are a refractive index along an in-plane slow axis, arefractive index along an in-plane fast axis, and a refractive indexalong a thickness direction, respectively, of the second opticalcompensation layer; an angle between the in-plane slow axis of the firstoptical compensation layer and the first optical axis is within ±2degrees, and the second optical compensation layer has an optical axissubstantially normal to a surface of second substrate; and the first andsecond optical compensation layers have in-plane retardations N₁ (nm)and N₂ (nm), respectively, satisfying the following relationship;36.859+7.617N ₂ ≦N ₁≦168.193+9.783N ₂in a range of 0<N₂≦6.0.

The present invention also provides, in a fifth aspect thereof, an IPSLCD device including: a liquid crystal (LC) layer having a first opticalaxis; first and second substrates sandwiching therebetween the LC layer,the first and second substrates being disposed on a light-emitting sideand a light-incident side, respectively, of the LCD device; first andsecond polarizing films sandwiching therebetween the first and secondsubstrates and the LC layer, the first and second polarizing filmshaving polarization axes extending normal to each other and beingdisposed on the light-emitting side and the light-incident side,respectively, of the LCD device, each of the first and second polarizingfilms including a first protective layer, a polarization layer and asecond protective layer, which are disposed consecutively as viewed fromthe light-incident side, each of the first and second protective layershaving a retardation in a thickness direction thereof; and an opticalcompensation layer disposed between the first polarizing film and thefirst substrate, wherein: the optical compensation layer has abirefringence satisfying the relationship 0.0≦(ns−nz)/(ns−nf)≦0.5, wherens, nf and nz are a refractive index along an in-plane slow axis, arefractive index along an in-plane fast axis, and a refractive indexalong a thickness direction, respectively, of the optical compensationlayer; an angle between the in-plane slow axis of the opticalcompensation layer and the first optical axis is within ±2 degrees; andthe optical compensation layer has an in-plane retardation N₁ (nm), andthe second protective layer of the second polarizing film has aretardation R_(t2):

$R_{t2} = {\left( {\frac{{npx}_{2} + {npy}_{2}}{2} - {npz}_{2}} \right) \times d_{2}}$in the thickness direction thereof, where npx₂, npy₂, npz₂ and d₂ are arefractive index along an in-plane slow axis, a refractive index alongan in-plane fast axis, a refractive index along an orthogonal axis and athickness, respectively, of the second protective layer of the secondpolarizing film; and the N₁ and R_(t2) satisfy therebetween thefollowing relationship:83.050−1.18R _(t2) ≦N ₁≦228.090−1.08R _(t2).in a range of 0≦R_(t2)≦55 nm.

The present invention also provides, in a sixth aspect thereof, apolarizing film pair for use in a liquid crystal display (LCD) device,including: a first polarizing film including a first protective layer, afirst polarization layer having a first absorption axis, a firstretardation film having an optical axis normal to a surface of the firstpolarization layer, the first retardation film having anegative-single-axis birefringence having an in-plane retardation in arange of 0 to 15 nm and an orthogonal retardation of 50 to 123 nm, and asecond retardation film having an optical axis parallel to the surfaceof the polarization layer and the first absorption axis, the secondretardation film having a negative uniaxial birefringence having anin-plane retardation of 83 to 210 nm, the second retardation film havingdifferent refractive indexes (no) and (ne) along in-plane optical axesextending parallel to each other, and a refractive index (nz) along adirection normal to the surface of the polarization layer, therefractive indexes satisfying the relationship no=nz>ne, the firstprotective layer, the first polarization layer, the first retardationfilm and the second retardation film being consecutively layered; and asecond polarizing film including a second protective layer, a secondpolarization layer having a second absorption axis, and a thirdretardation film having a birefringence having an in-plane retardationof 0 to 10 nm and an orthogonal retardation of 0 to 35 nm, the secondprotective layer, the second polarization layer and the thirdretardation layer being consecutively layered.

The present invention also provides, in a seventh aspect thereof, an LCDdevice including a liquid crystal (LC) layer having a homogeneousorientation, a pair of substrates sandwiching therebetween the LC layer,the polarizing film pair of the present invention sandwichingtherebetween the substrates and the LC layer.

In accordance with the IPS LCD device of the present invention, theleakage light leaking upon display of black in the slanted viewingdirection can be reduced to improve the contrast ratio of the LCDdevice, and the chromaticity shift between the normal viewing directionand a slanted viewing direction can be suppressed to thereby improve theimage quality of the IPS LCD device.

The above and other objects, features and advantages of the presentinvention will be more apparent from the following description,referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an IPS LCD device according to a firstembodiment of the present invention.

FIGS. 2A and 2B are enlarged partial sectional views of the IPS LCDdevice of FIG. 1.

FIG. 3 is a perspective view of the LCD device, shown for defining theazimuth angle φ and the viewing angle θ of the LCD device.

FIG. 4 is a perspective view of an optical compensation layer, shown fordefining the retardation thereof.

FIG. 5 is graph showing the relationship between the in-planeretardation of the optical compensation layer and the transmittance inthe slanted viewing direction.

FIG. 6 is a graph showing the relationship between the in-planeretardation of the optical compensation layer and the chromaticity shiftin the slanted viewing direction.

FIG. 7 is a graph showing a range of superior combinations of theretardation of the optical compensation layer and the thickness of theprotective layer.

FIGS. 8A and 8B are partial sectional views of an IPS LCD deviceaccording to a second embodiment of the present invention.

FIG. 9 is a graph showing the relationship between the in-planeretardation of the optical compensation layer on the CF-substrate sideand the normalized transmittance in the slanted viewing direction.

FIG. 10 is a graph showing the relationship between the in-planeretardation of the optical compensation layer on the CF-substrate sideand the normalized chromaticity shift in the slanted viewing direction.

FIG. 11 is a graph showing a range of superior combinations of theretardations of the first optical compensation layer and the secondoptical compensation layer.

FIG. 12 is graph showing the relationship between the in-planeretardation of the optical compensation layer on the CF-substrate sideand the normalized transmittance in the slanted viewing direction.

FIG. 13 is a graph showing the relationship between the in-planeretardation of the optical compensation layer on the CF-substrate sideand the normalized chromaticity shift in the slanted viewing direction.

FIG. 14 is a graph showing a range of superior combinations of theretardations of the first optical compensation layer and the secondoptical compensation layer.

FIG. 15 is a graph showing the relationship between the in-planeretardation of the optical compensation layer on the CF-substrate sideand the normalized transmittance in the slanted viewing direction.

FIG. 16 is a graph showing the relationship between the in-planeretardation of the optical compensation layer on the CF-substrate sideand the normalized chromaticity shift in the slanted viewing direction.

FIG. 17 is a graph showing a range of superior combinations of theretardations of the first optical compensation layer and the secondoptical compensation layer.

FIG. 18A is a schematic sectional view showing the IPS LCD device of thefourth embodiment, and FIGS. 18B to 18D are refractive index ellipses ofthe optical compensation layer shown in FIG. 18A.

FIG. 19 is a sectional view of a conventional IPS LCD device describedin a publication.

FIG. 20 is an exploded perspective view of another conventional IPS LCDdevice described in another publication.

FIG. 21 is a graph showing the relationship between the combination ofthe retardations of the second protective layer and the third protectivelayer and the normalized leakage light in the slanted viewing direction,in a specific condition of the refractive index of the opticalcompensation layer.

FIG. 22 is a graph showing the relationship between the combination ofthe retardations of the second protective layer and the third protectiveand the normalized leakage light in the slanted viewing direction, inanother specific condition of the refractive index of the opticalcompensation layer.

FIG. 23 is a graph showing the relationship between the combination ofthe retardations of the second protective layer and the third protectiveand the normalized leakage light in the slanted viewing direction, inanother specific condition of the refractive index of the opticalcompensation layer.

FIG. 24 is a graph showing the relationship between the retardation ofthe second protective layer in the thickness direction and thenormalized leakage light in the slanted viewing direction.

FIG. 25 is a graph showing the relationship between the retardation ofthe third protective layer in the thickness direction and the normalizedleakage light in the slanted viewing direction.

FIG. 26 is a graph showing the relationship between the retardation ofthe second protective layer in the thickness direction and thenormalized chromaticity shift in the slanted viewing direction.

FIG. 27 is a graph showing the relationship between the combination ofthe retardations of second protective layer and the third protectivelayer in the thickness direction and the normalized chromaticity shiftin the slanted viewing direction.

FIG. 28 is a graph showing a range of superior combinations of theretardations of the optical compensation layer and the protective layerand the retardation of the protective layer both in the thicknessdirection.

PREFERRED EMBODIMENT OF THE INVENTION

Now, the present invention is more specifically described with referenceto accompanying drawings, wherein similar constituent elements aredesignated by similar reference numerals throughout the drawings.

Referring to FIG. 1, an IPS LCD device, generally designated by numeral100, according to a first embodiment of the present invention includes alight-incident-side polarizing film 101, a TFT (thin-film-transistor)substrate 102, an LC layer 103, a CF (color-filter) substrate 104, and alight-emitting-side polarizing film 105, which are consecutivelyarranged as viewed along the travelling direction of the backlight. Anorientation film 111 is interposed between the LC layer 103 and the TFTsubstrate 102, and another orientation film 113 is interposed betweenthe LC layer 103 and the CF substrate 104. An optical compensation layer117 having a specific optical characteristic is interposed between theCF substrate 104 and the light-emitting-side polarizing film 105. Theoptical compensation layer 117 may be provided by bonding or coating,for example.

The TFT substrate 102 includes a glass substrate body 106, insulationfilms 107, and a plurality of pixels each including a TFT 108, a pixelelectrode 109 and a portion of a counter electrode 110. Each TFT 108controls the potential of a corresponding pixel electrode 109. The pixelelectrode 109 and a corresponding portion of the counter electrode 110apply therebetween a lateral electric field onto the LC molecules 112 inthe LC layer 103. The insulation films 107 include an organic film and asilicon nitride film. The CF substrate 104 includes color filters 114, ashield film pattern 115 and a glass substrate body 116, The colorfilters 114 add three primary colors to the light passed by the LC layer103. The shield film pattern 115 shields TET 108, data lines etc.against the light.

FIG. 2A shows the detail of a portion of the LCD device 100 of FIG. 1,including the light-emitting-side polarizing film 105, the opticalcompensation layer 117 and the glass substrate body 116 of the CFsubstrate 104. FIG. 2B shows the detail of another portion of the LCDdevice of FIG. 1, including the light-incident-side polarizing film 101and the glass substrate body 106 of the TFT substrate 102.

The light-incident-side polarizing film 101 includes, as shown in FIG.2B, a polarization layer 120 made of PVA, for example, and first andsecond protective layers 121 and 122 made of TAC, for example, andsandwiching therebetween the polarization layer 120. Thelight-emitting-side polarizing film 105 includes, as shown in FIG. 2A, apolarization layer 120 and third and fourth protective layers 123 and124 sandwiching therebetween the polarization layer 120. Each protectivelayer 121, 122, 123 or 124 acts as a retardation layer having a negativesingle optical axis, and has a retardation depending on the thickness ofthe each protective layer.

The inventors conducted simulations on the IPS LCD device 100 having theabove structure, to obtain the suitable conditions as to the opticalcharacteristics of the optical compensation layer 117 including theretardation thereof, and the optical characteristics of the secondprotective layer 122 of the light-incident-side polarizing film 101. Thesuitable conditions as used herein are such that the leakage light andthe chromaticity shift are reduced to satisfactory levels when viewingthe LCD device in the slanted viewing direction. The satisfactory levelfor the leakage light is such that a general observer does not perceivethe leakage light as a disturbing phenomenon, whereas the satisfactorylevel for the chromaticity shift is such that the chromaticity shift isnot increased compared to a general IPS LCD device including no opticalcompensation layer such as 117.

In the simulations, parameters of the components other than the opticalcompensation layer 117 and the second protective layer 122 of thelight-incident-side polarizing film 101 are fixed as tabulated in Table1.

TABLE 1 refractive index optical axis thickness (μm) (or birefringence)(φ, θ) fourth protective 80 (0.0015) (0, 90) layer 124 polarizationlayer — — (90, 0) 120 third protective 80 (0.0015) (0, 90) layer 123glass substrate body — 1.54 — 116 CF substrate 114 — 1.5 — LC layer 103Δnd = 300 ± 80 nm insulation (organic) — 1.5 — layer 107 insulation(SiNx) — 1.9 — layer 107 insulation (SiO₂) — 1.48 — layer 107 glasssubstrate body — 1.54 — 106 polarization layer — — (0, 0)  120 firstprotective layer 80 (0.0015) (0, 90) 121

In the notation of Table 1, as shown in FIG. 3, the optical axis isrepresented in the XYZ-coordinate system by an azimuth angle φ and aninclined angle θ, wherein X-axis and Y-axis are parallel to thepolarization axes of the polarizing films. The azimuth angle φ of anarbitrary vector is represented by an angle between the X-axis and thearbitrary vector projected onto the X-Y plane, and the inclined angle isrepresented by the angle between the arbitrary vector and the X-Y plane.

An experiment was conducted in an IPS LCD device prior to thesimulations to obtain a brightness level of the backlight at which theleakage light in the slanted viewing direction does not substantiallydegrade the image quality, after gradually reducing the brightness levelof the backlight. The experiment revealed that a half of the maximumbrightness level generally used for the image display allowed theleakage light not to significantly affect the image quality in theslanted viewing direction, and a quarter of the maximum brightness levelallowed the leakage light not to be perceived by the observer. In viewof these results, half of the leakage light of a typical IPS LCD devicein the slanted viewing direction is used as a reference leakage level,at which a suitable image quality is achieved. The slanted viewingdirection is set at an azimuth angle, which is 45 degrees away from thepolarization axis of the polarizing film.

As to the retardation of the optical compensation layer, as shown inFIG. 4, the in-plane retardation is defined herein by (ns−nf)d and theorthogonal retardation (Δnd), i.e., retardation in the thicknessdirection, is herein defined by:

$\left( {\frac{{nf} + {ns}}{2} - {nz}} \right) \times d$where ns, nf and nz are the refractive index along the in-plane slowaxis, the refractive index along the in-plane fast axis and therefractive index in the thickness direction, and where d is theequivalent thickness of the optical compensation layer. In thesimulations, an optical compensation layer 117 having an opticalcharacteristic of (ns−nz)/(ns−nf)≦0.5 was employed, whereas a secondprotective layer 122 having an optical characteristic of(ns−nz)/(ns−nz)≧6 was employed. The optical axis of the secondprotective layer 122 was directed to (φ, θ)=(0,90). The angle betweenthe slow axis of the optical compensation layer 117 and the optical axisof the LC layer 103 is preferably set within ±2 degrees, and morepreferably set at zero degree.

FIG. 5 shows the relationship between the in-plane retardation of theoptical compensation layer (OPL) 117 and the normalized transmittance ofthe LCD device of the present embodiment in the slanted viewingdirection, plotted for different length of the second protective layer(SPL) 122. The transmittance shown in the graph is evaluated in anazimuth angle of 45 degrees and in a viewing angle of 70 degrees upondisplay of black, and is normalized by the transmission of aconventional LCD device obtained in the same conditions. Theconventional LCD device is similar to the LCD device of the presentembodiment except that the conventional LCD device has therein nooptical compensation layer.

A satisfactory level of the leakage light upon display of black can beobtained if the normalized transmittance is 0.5 or lower, as describedbefore. From FIG. 5, the normalized transmittance of 0.5 or lower can beobtained, if the thickness of the second protective layer 122 is withinthe range of 0 to 80 μm, and the retardation of the optical compensationlayer 117 is within the range of 45 to 225 nm. More specifically, asuitable combination of this range of the thickness of the secondprotective layer 122 and this range of the retardation of the opticalcompensation layer 117 provides the satisfactory level of the leakagelight upon display of black. The range of thickness, 0 to 80 μm, of thesecond protective layer 122 corresponds to an in-plane retardation of 0to 6.0 nm of the second protective layer 122, and a retardation(orthogonal retardation) of 0 to 55 nm in the thickness directionthereof.

FIG. 6 shows the relationship between the in-plane retardation of theoptical compensation layer (OPL) 117 and the chromaticity shift fordifferent thicknesses of the SPL 122. The chromaticity shift isnormalized by the chromaticity shift measured in the conventional LCDdevice having therein no optical compensation layer. The chromaticityshift is defined in a chromaticity coordinate system using thechromaticity, (u′,v′)=(u₀′,v₀′), as observed in the normal viewingdirection where (φ,θ)=(0, 0), and the chromaticity, (u′, v′)=(u₁′, v₁′),as observed in the slanted viewing direction where (φ, θ)=(45, 70), asfollows:Δu′v′=√{square root over ((u ₁ ′−u ₀′)²+(v ₁ ′−v ₀′)²)}{square root over((u ₁ ′−u ₀′)²+(v ₁ ′−v ₀′)²)}.In short, the chromaticity shift represents a difference between thecolor observed in the normal direction (direction normal to the screen)and the color observed in the slanted direction. The definition of thechromaticity shift appears in “1976 CIE Chromaticity Diagram”.

In FIG. 6, assuming that the second protective layer 122 has a thicknessbetween 0 μm and 80 μm, the normalized chromaticity shift remains “1” orlower if the retardation of the optical compensation layer 117 is withina range between 30 nm and 230 m nm, thereby achieving an effectivesuperior suppression of the chromaticity shift. It will be alsounderstood that, if a configuration, wherein the second protective layer122 has a thickness between 0 μm and 40 μm and the retardation of theoptical compensation layer 117 is 130 nm or above, is employed, a moresuperior suppression of the chromaticity shift can be obtained.

A combination of the thickness of the second protective layer 122 andthe retardation of the optical compensation layer 117, which achieves asatisfactory level of the leakage light in the slanted viewingdirection, can be obtained from FIGS. 5 and 6. The resultant combinationis shown in FIG. 7, wherein the superior combination of the retardationof the OPL 117 and the thickness of the second protective layer (SPL)122 is represented by the dark area. The dark area can be expressed bythe formula using approximating linear equations for the upper and lowerlimits of the dark area, as follows:83.050−0.810x≦y≦22.8090−0.74x, and0<x≦80,where x and y represent the thickness of the second protective layer 122and the retardation of the optical compensation layer 117.

In the present embodiment, the combination expressed by the aboveformula and represented by the dark area in FIG. 6 provides asatisfactory level for the leakage light in the slanted direction and asuperior suppression of the chromaticity shift. This is consideredbecause the optical dispersions generated in the second protective layer122 of the polarizing film 101 on the light-incident side, LC layer is103 and CF substrate 104 can be suppressed by the optical compensationlayer 117, thereby achieving a less-dispersed state of light on thesurface of the polarization layer 120 of the polarizing film 105 on thelight-emission side.

FIGS. 8A and 8B show, similarly to FIGS. 2A and 2B, partial sectionalviews of an LCD device according to a second embodiment of the presentinvention. The LCD device of the present embodiment includes anotheroptical compensation layer 118 between the optical compensation film 101and the glass substrate body 106 in addition to the configuration of theLCD device of the first embodiment.

Simulations were conducted for the LCD device of Fig. to obtain theconditions for achieving a satisfactory level for the leakage light inthe slanted viewing direction, and a satisfactory level for thechromaticity shift compared to the conventional IPS LCD device having nooptical compensation layer 117 or 118. In these simulations, thefollowing values for the parameters of the other components were used.

TABLE 2 refractive index optical axis thickness (μm) (or birefringence)(φ, θ) fourth protective 80 (0.0015) (0, 90) layer 124 polarizationlayer — — (90, 0) 120 third protective layer 80 (0.0015) (0, 90) 123glass substrate body 700 1.54 — 116 CF substrate 0.1 1.5 — 114 LC layerΔnd = 300 ± 80 nm 103 insulation (organic) — 1.5 — layer 107 insulation(SiNx) — 1.9 — layer 107 insulation (SiO₂) — 1.48 — layer 107 glasssubstrate body — 1.54 — 106 second protective 80 (0.0015) (0, 90) layer122 optical compensation — — (0, 0)  layer120 first protective layer 80(0.0015) (0, 90) 121

In the simulations, the optical compensation layers 117 and 118 had anoptical characteristic wherein:(ns−nz)/(ns−nf)≦0.5.The angle between the slow axis of the optical compensation layer 117 onthe CF-substrate side and the optical axis of the LC layer 103 should beset within ±2 degrees, and preferably set at zero degree.

FIG. 9 shows the relationship between the retardation of the opticalcompensation layer 117 on the CF-substrate side and the normalizedtransmittance of the LCD device in the slanted viewing direction. InFIG. 9, if the retardation of the optical compensation layer 118 on theTFT-substrate side is within a range between 0.6 nm and 46 nm, and theretardation of the optical compensation layer 117 on the CF-substrateside is within a range between 30 nm and 180 nm, the normalizedtransmittance is 0.5 or above. That is, the combination of theseretardations provides a satisfactory level for the leakage light in theslanted viewing direction light upon display of black.

FIG. 10 shows the relationship between the in-plane retardation of theoptical compensation layer 117 on the CF-substrate side and thenormalized chromaticity shift in the slanted viewing direction. In FIG.10, if the retardation of the optical compensation layer 118 is within arange between 0.6 nm and 45 nm, the normalized chromaticity shift is “1”or lower irrespective of the retardation of the optical compensationlayer 117 on the CF-substrate side. In addition, if the opticalcompensation layer 117 on the CF-substrate side is 150 nm or above, thenormalized chromaticity shift is 0.8 or lower, to achieve a moreeffective suppression of the chromaticity shift.

By using the relationships shown in FIGS. 9 and 10, a combination of theoptical compensation layers 117 and 118 can be obtained, which achievesa satisfactory level for the leakage light and effectively suppressesthe chromaticity shift. The resultant combination is shown by the darkarea in FIG. 11, wherein x and y represent the retardation of theoptical compensation layer 118 on the TFT-substrate side, and theretardation of the optical compensation layer 117 on the CF-substrateside. The lower limit and the upper limit of the dark area in FIG. 11can be approximated by third-order equations of x, and the dark area isdefined by the following relationship:29.87+1.79x−0.048x ²+0.001x ³ ≦y≦187.22−1.66x+0.0475x ²−0.0009x ³in the range of 0.6≦x≦46.

In the present embodiment, by setting the in-plane retardations of theoptical compensation layers 117 and 118 within the range shown by thedark area in FIG. 11, satisfactory levels for the leakage light and thechromaticity shift can be obtained. The optical compensation layer 118may be adhered onto the second protective layer 122, as in theconfiguration of the present embodiment.

An LCD device according to a third embodiment of the present inventionis similar to the LCD device of the second embodiment, except that theangle between the slow axis of the optical compensation layer 118 on theTFT-substrate side and the optical axis of the LC layer 103 is setwithin a range of ±2 degrees, preferably at zero degree in the presentembodiment.

Simulations of the LCD device as to the optical characteristics of theoptical compensation layers 117 and 118 including the retardationsthereof were conducted for achieving a satisfactory level for theleakage light in the slanted viewing direction and suppressing thechromaticity shift down to the level of the conventional IPS LCD device.

FIG. 12 shows the relationship between the in-plane retardation of theoptical compensation layer 117 on the CF-substrate side and thenormalized transmittance in the slanted viewing direction. In FIG. 12,it will be understood that there is a suitable combination ofretardations, wherein the retardations of the optical compensationlayers 117 and 118 are within ranges between 0.6 nm and 92 nm andbetween 60 nm and 230 m nm, respectively. This combination provides anormalized transmittance of 0.5 or lower whereby a satisfactory levelfor the leakage light in the slanted viewing direction is achieved upondisplay of black.

FIG. 13 shows the relationship between the in-plane retardation of theoptical compensation layer 117 on the CF-substrate side and thenormalized chromaticity shift in the slanted viewing direction. In FIG.13, it will be understood that the normalized chromaticity shift is “1”or lower if the retardation of the optical compensation layer 117 on theCF-substrate side is above about 140 nm for the case where theretardation of the optical compensation layer 118 on the TFT-substrateside is within a range between 0.6 mm and 15 nm. It will be alsounderstood that the normalized chromaticity shift is suppressed down to“1” or lower if the retardation of the optical compensation layer 118 onthe TFT-substrate side is about 15 nm or above irrespective of theretardation of the optical compensation layer 117 on the CF-substrateside, and that the normalized chromaticity shift is 0.8 or lower if theretardation of the optical compensation layer 117 on the CF-substrateside is about 150 nm or above.

By using the relationships shown in FIGS. 12 and 13, a combination ofthe optical compensation layers 117 and 118 can be obtained, whichachieves a satisfactory level for the leakage light and effectivelysuppresses the chromaticity shift. The resultant combination is shown bythe dark area in FIG. 14, wherein x and y represent the retardation ofthe optical compensation layer 118 on the TFT-substrate side, and theretardation of the optical compensation layer 117 on the CF-substrateside, respectively.

The dark area is divided into three ranges of x for 0.6≦x≦22, 22<x≦62,and 62≦92, and the lower limit and the upper limit of each range areapproximated by two-order to five-order equations for defining the darkarea of FIG. 14. The resultant ranges are defined as follows.

(i) in the range of 0.6≦x≦22:162.560−8.874x+2.258x ²−0.291x ³+0.0165x ⁴−0.000346x ⁵≦y≦142.465+2.546x−0.017x ²(ii) in the range of 22<x≦62:73.04−0.977x+0.0220x ² ≦y≦142.465+2.546x−0.017x ²(iii) in the range of 62<x≦92:73.04−0.977x+0.00220x2≦y≦1205.596−41.304x+0.586x ^(2−0.0028) x ³

In the present embodiment, by setting the in-plane retardations of theoptical compensation layers 117 and 118 within the range shown by thedark area in FIG. 14, a satisfactory level for the leakage light andsuppression of the chromaticity shift can be obtained. It is to be notedthat these retardations should be set within the dark area while settingthe angle between the slow axis of the optical compensation layer 118 onthe TFT-substrate side and the optical axis of the LC layer 103 within±2 degrees,

An IPS LCD device according to a fourth embodiment of the presentinvention has a configuration similar to that of the LCD device of thesecond embodiment except that the optical compensation layer 118 has anoptical characteristic of:(ns−nz)/(ns−nf)≦−2in the present embodiment and that the optical compensation layer 118has an optical axis normal to the substrate surface in the presentembodiment. The optical compensation layer 118 has a positive singleaxis and a retardation of about 0 to 55 nm in the thickness directionthereof. Simulations were conducted to the LCD device of the presentembodiment, to obtain a satisfactory level for the leakage light andsuppression of the chromaticity shift down to the chromaticity shift ofthe conventional IPS LCD device having a configuration similar to theLCD device of the present embodiment except for absence of the opticalcompensation layers 117 and 118.

FIG. 15 shows the relationship obtained by simulations between thein-plane retardation of the optical compensation layer 117 on theCF-substrate side and the normalized transmittance in the slantedviewing direction. In FIG. 15, it will be understood that there is acombination of retardations, is which achieves a normalizedtransmittance of 0.5 or lower to obtain a satisfactory level for theleakage light in the slanted viewing direction upon display of black.The combination is such that the retardations of the opticalcompensation layers 117 and 118 are within ranges between 0 nm and 6.0nm and between 45 nm and 225 nm, respectively.

FIG. 16 shows the relationship between the in-plane retardation of theoptical compensation layer 117 on the CF-substrate side and thenormalized chromaticity shift in the slanted viewing direction. In FIG.16, it will be understood that the normalized chromaticity shift is “1”or lower if the retardation of the optical compensation layer 117 iswithin a range between 30 nm and 230 nm for the case where theretardation of the optical compensation layer 118 on the TFT-substrateside is within a range between 0 nm and 6.0 nm. It will be alsounderstood that the normalized chromaticity shift is suppressed down to“1” or lower if the retardation of the optical compensation layer 118 onthe TFT-substrate side is 3.0 nm and 6.0 nm and the retardation of theoptical compensation layer 117 on the CF-substrate side is 130 nm orabove.

By using the relationships shown in FIGS. 15 and 16, a combination ofthe retardations of the optical compensation layers 117 and 118 can beobtained, which achieves a satisfactory level for the leakage light andeffectively suppresses the chromaticity shift. The resultant combinationis shown by the dark area in FIG. 17, wherein x and y represent theretardation of the optical compensation layer 118 on the TFT-substrateside and the retardation of the optical compensation layer 117 on theCF-substrate side, respectively.

The lower limit and upper limit of the dark area are approximated bylinear equations of x for defining the dark area of FIG. 17. The rangeof combination is defined as follows:36.859+7.617x≦y≦168.193+9.783xin the range of 0<x≦6.0.

FIG. 18A shows a schematic sectional view of an IPS LCD device of thepresent embodiment, whereas FIGS. 18B to 18D show the principle of theoptical compensation effected therein. The optical compensation layer118 on the TFT substrate 101 can be represented by a refractive-indexellipsoid having a longer axis normal to the substrate surface, whereasthe second protective layer 122 of the light-incident-side polarizingfilm 101 can be expressed by a refractive-index ellipsoid having alonger axis parallel to the substrate surface.

If the optical compensation layer 118 is observed as a separate layer inthe slanted viewing direction “a” as shown in FIG. 18A, the refractiveindex of the optical compensation is layer 118 is observed to be reducedin the lateral direction as shown in FIG. 18B. If the second protectivelayer 122 is observed as a separate layer in the direction “a”, therefractive index of the second protective layer 122 is observed to beextended in the lateral direction, as shown in FIG. 18C. By layering theoptical compensation layer 118 and the second protective layer 122 oneon another, the refractive index of the second protective layer 122extended in the lateral direction is compensated by the refractive indexof the optical compensation layer 118 reduced in the lateral direction,whereby the total refractive-index ellipsoid is observed to be morelikely to a circle, as shown in FIG. 18D.

In the present embodiment, the in-plane retardations of the opticalcompensation layers 117 and 118 are determined to satisfy the dark areashown in FIG. 17, to obtain a satisfactory level for the leakage lightin the slanted viewing direction and suppression of the chromaticityshift. In the first embodiment, the thickness of the second protectivelayer 122 is reduced to lower the retardation thereof and thus suppressthe chromaticity shift, whereas in the present embodiment, the opticalcompensation layer 118 having a positive single optical axis compensatesthe retardation of the second protective layer 122 having a negativesingle optical axis to suppress the chromaticity shift. The lattercompensation in the present embodiment corresponds to the compensationachieved by reducing the thickness of the second protective layer 122 inthe first embodiment.

An IPS LCD device according to a fifth embodiment of the presentinvention has a configuration similar to that of the LCD device 100 ofthe first embodiment shown in FIG. 1. In the present embodiment, theleakage light in the slanted viewing direction is suppressed by acombination of the retardation in the thickness direction of the secondprotective layer 122 (FIG. 2B) of the light-incident-side polarizingfilm 101 and the retardation in the thickness direction of the thirdprotective layer 123 (FIG. 2A) of the light-emitting-side polarizingfilm 105. In the present embodiment, the “thickness d” of the secondprotective layer 122 of the light-incident-side polarizing film 101 inthe first embodiment is replaced by the “retardation” of the secondprotective layer 122 in the thickness direction. By replacing xrepresenting the “thickness d” of the second protective layer 122 inFIG. 7 by the “retardation x” of the second protective layer 122, acombination of the retardations x and y of the second protective layer122 and the optical compensation layer 118 can be obtained, whichachieves a satisfactory level for the leakage light and suppression ofthe chromaticity shift. The combination achieving a normalizedtransmittance of 0.5 is represented by approximating the lower limit andthe upper limit of the dark area in FIG. 7 by a linear function of x, toobtain the following relationship:82.813−0.900x≦y≦229.604−1.214xwhere 0 nm<x≦55 nm corresponding to 0 μm<d≦80 μm.

Simulations were conducted for the LCD device of FIG. 2 including anoptical compensation layer 117 having the retardation “y” in the aboveformula, to obtain a combination of the retardation of the secondprotective layer 122 in the thickness direction and the retardation ofthe third compensation layer 123 in the thickness direction, whichachieves a further normalized leakage light of “1” or lower. The furthernormalized leakage light is defined herein assuming that the minimumnormalized transmittance, 0.3, for the case where the second protectivelayer 122 has a thickness of 80 μm (corresponding to a retardation of 55nm) assumes “1”. These simulations were conducted for the cases wherethe optical compensation layer 117 has a refractive index satisfying0.0≦(ns−nz)/(ns−nf)<0.2, a refractive index satisfying0.2≦(ns−nz)/(ns−nf)<0.4 and a refractive index satisfying0.4≦(ns−nz)/(ns−nf)≦0.5. The other parameters used in the simulationsare tabulated in Table 3

TABLE 3 refractive index optical axis thickness (μm) (or birefringence)(φ, θ) fourth protective 80 (0.0015) (0, 90) layer 124 polarizationlayer — — (90, 0) 120 glass substrate body 1.54 — 116 CF substrate 1141.5 — LC layer 103 Δnd = 300 ± 80 nm insulation (organic) — 1.5 — layer107 insulation (SiNx) — 1.9 — layer 107 insulation (SiO₂) — 1.48 — layer107 glass substrate body — 1.54 — 106 polarization layer — — (0, 0)  120first protective layer 80 (0.0015) (0, 90) 121

The retardation R_(t2) (nm) of the second protective layer 122 in thethickness direction is defined by the following relationship:

$R_{t2} = {\left( {\frac{{npx}_{2} + {npy}_{2}}{2} - {npz}_{2}} \right) \times d_{2}}$where npx₂, npy₂, npz₂ and d₂ are the maximum in-plane refractive index,the in-plane refractive index in the direction normal to the directionof the maximum refractive index, the orthogonal refractive index and thethickness, respectively, of the second protective layer 122.

The retardation R_(t3) (nm) of the third protective layer 123 in thethickness direction is defined by the following relationship:

$R_{t3} = {\left( {\frac{{npx}_{3} + {npy}_{3}}{2} - {npz}_{3}} \right) \times d_{3}}$where npx₃, npy₃, npz₃ and d₃ are the maximum in-plane refractive index,the in-plane refractive index in the direction normal to the directionof the maximum refractive index, the orthogonal refractive index and thethickness, respectively, of the third protective layer 123.

The in-plane retardation of the second protective layer 122 is definedby (npx₂−npy₂)×d₂, whereas the in-plane retardation of the thirdprotective layer 123 is defined by (npx₃−npy₃)×d₃. In the simulations,the second protective layer 122 and the third protective layer hadin-plane retardations equal to or smaller than 10 nm.

FIG. 21 shows the relationship between the normalized leakage light andthe combination of the retardations (Δnd) of the second protective layer(SP1) 122 and the third protective layer (TPL) 123 both in the thicknessdirection for the optical characteristic where the refractive index ofthe optical compensation layer satisfies 0.0≦(ns−nz)/(ns−nf)<0.2. Thedark area in FIG. 21 provides a combination of the retardations of thesecond protective layer 122 and the third protective layer 123, whichachieves a normalized leakage light of “1” or lower in the slantedviewing direction. The dark area is divided into two ranges of x where0.0≦x<37.5 and 37.5≦x≦55. The upper limit and the lower limit of “y” ineach divided range of the dark area can be approximated by second-orderequations of “x”, where x and y represent the retardations Rt2 and Rt3,respectively, of the second protective layer 122 and the thirdprotective layer 123. The dark area can be expressed by the followingrelationships:

(i) in the range of 0.0≦x<37.5:48.3−1.05x+0.00952x ² ≦y≦111.0−0.529x−0.00472x ²(ii) in the range of 37.5≦x≦55.0:34.2−16.9x−0.222x ² ≦y≦111.0−0.529x−0.00472x ²

FIG. 22 shows the relationship between the normalized leakage light andthe combination of the retardations (Δnd) of the second protective layer(SPL) 122 and the third protective layer (TPL) 123 both in the thicknessdirection for the optical characteristic where the refractive index ofthe optical compensation layer 117 satisfies 0.2≦(ns−nz)/(ns−nf)<0.4.The dark area in FIG. 22 provides a combination of the retardations ofthe second protective layer 122 and the third protective layer 123,which achieves a normalized leakage light of “1” or lower in the slantedviewing direction. The dark area is divided into three ranges of x where0.0≦x<10.0, 10.0≦x<38.0 and 38.0<x≦55. The upper limit and the lowerlimit of “y” in each divided range of the dark area can be approximatedby second-order equations of “x” where x and y represent theretardations Rt2 and Rt3, respectively, of the second protective layer122 and the third protective layer 123. The dark area can be expressedby the following relationships:

(i) in the range of 0.0≦x<10.0:8.75−0.957x+0.0093x ² ≦y≦90.3−0.368x−0.00832x ²;(ii) in the range of 10.0≦x<38.0:0≦y≦90.3−0.368x−000832x ^(2;); and(iii) in the range of 38.0≦x≦55.0:431.0−22.8x+0.302x ²≦y≦90.3−0.368x−0.00832x ².

FIG. 23 shows the relationship between the normalized leakage light andthe combination of the retardation (x) of the second protective layer122 and the retardation (y) 0f the third protective layer 123 for theoptical compensation layer 117 having an optical characteristicsatisfying 0.4≦(ns−nz)/(ns−nf)≦05. The dark area in FIG. 23 provides asatisfactory level, 1.0, for the normalized leakage light. By using asimilar approximating equation, the dark area in FIG. 23 can beexpressed by the following relationship:0≦y≦65.2−0.805xin the range of 0.0≦x≦55.0.

In the present embodiment, by using the combination of the retardationsof the second protective layer 122 and the third protective layer 123both in the thickness direction and satisfying the dark areas shown inFIGS. 21, 22 and 23 for the respective cases of the refractive indexesof the optical compensation layer 117, a satisfactorily low opticaldispersion can be obtained at the polarization layer 120 of thelight-emitting-side polarizing film 105. Thus, the IPS LCD device of thepresent embodiment achieves more reduced leakage light in the slantedviewing angle upon display of black compared to the LCD device of thefirst embodiment. Moreover, if the second protective layer 122 has aretardation within a range of 0 to 25 nm, the normalized leakage lightis further reduced down to as low as 0.5 or lower depending on theretardation of the third protective layer 123.

It is to be noted that even the absence of the second protective layer122 in the present embodiment achieves an advantage similar to that ofthe case where the second protective layer 122 has a retardation Rt2 ofzero.

An IPS LCD device according to a sixth embodiment of the presentinvention has a configuration similar to that of the LCD device 100 ofthe first embodiment except for the parameters of the second protectivelayer 122 and the third protective layer 123. In the simulations of thepresent embodiment, the third protective layer 123 had an in-planeretardation of 0 to 10 nm, and the second protective layer 122 had anin-plane retardation of 0 to 8 nm. The optical axes of the secondprotective layer 122 and the third protective layer 123 were normal tothe substrate surface. A negative uniaxial retardation film was used asthe optical compensation layer 117, having a birefringence whereinno=n1=nz>ne=n2, given n1 and n2 being in-plane refractive indexes in thedirections normal to one another. The optical axis of the opticalcompensation layer 117 was parallel to the light-absorbing axis of thepolarization layer 120 (FIG. 2).

A polarizing film pair according to an embodiment of the presentinvention uses the principle of the polarizing films 105 and 101 shownin FIGS. 2A and 2B. The first polarizing film 105 of the polarizing filmpair of the present embodiment includes the protective layer 124, thepolarization layer 120, a first retardation film implementing thefunction of the protective layer 123, and a second retardation filmimplementing the function of the optical compensation layer 117. Thesecond polarizing film 101 includes the protective layer 121, thepolarization layer 120 and a third retardation film 122 implementing thefunction of the protective layer 122.

FIG. 24 shows the relationship obtained by the simulations between theretardation of the second protective layer 122 (third retardation film)and the normalized transmittance in the slanted viewing direction. Thenormalized transmittance in the slanted viewing direction changes asshown in FIG. 24 if the retardation of the second protective layer 122in the thickness direction is changed between 0 nm and 55 nm, with theretardation of the third protective layer (first retardation film) 123being fixed at 55 nm and 80 nm. The normalized transmittance, 0.25, forthe case where both the retardations of the second and third protectivelayers 122 and 123 are 55 nm is hereinafter used as a reference “1” as afurther normalized transmittance to show the change of the transmittancewhen the retardation of the third protective layer 123 is changedbetween 55 nm and 120 nm, with the retardation of the second protectivelayer 122 being fixed. The resultant further normalized transmittance isshown in FIG. 25.

It will be understood from FIG. 25 that the further normalizedtransmittance upon display of black can be reduced, if the retardationof the third protective layer is set within a range between 55 nm and123 nm, compared to the case where the retardation of the thirdprotective layer 123 is out of this range. This range of retardationachieves a lower level for the leakage light in the slanted viewingdirection upon display of black.

FIG. 26 shows the relationship obtained by the simulations between thenormalized chromaticity shift and the retardation of the secondprotective layer 122. In the simulations, the retardation of the secondprotective layer 122 is changed between 0 nm and 55 nm with theretardation of the third protective layer 123 being fixed at 55 nm and80 nm, to obtain the ratio of the chromaticity shift between the normalviewing direction and the slanted viewing direction. The resultant ratiois normalized by the ratio of the chromaticity shift between the normalviewing direction and the slanted viewing direction, which is obtainedfor the case where LCD device had no optical compensation layer, and thesecond and third protective layers had a thickness of 80 μm.

FIG. 27 shows the relationship between the normalized chromaticity shiftand the combination of the retardations of the second protective layer122 and the third protective layer 123 both in the thickness direction.In FIG. 27, the normalized chromaticity shift is “1” or lower if thesecond protective layer 122 has a retardation of 0 to 18 nm for the casewhere retardation of the third protective layer 123 in the thicknessdirection is in the range of 55 nm and 123 nm, which provides the abovesatisfactory level for the leakage light.

The dark area of normalized chromaticity shift shown in FIG. 27 isdefined hereinafter. The boundary (y) of the dark area of the normalizedchromaticity shift between 0.5 or lower and above 0.5 can be expressedby a second-order equation of x, where x and y represent theretardations of the second protective layer 122 and the third protectivelayer 123, respectively, both in the thickness direction. The dark areacan be defined by the following relationship:59.0−0.73x+0.34x ²≦y≦117.0−0.16x−0.27x ²in the range of 0<x≦17.

In other word, the range of y defined by the above formula achieves asatisfactory level for the normalized chromaticity shift, 0.5 or lower,thereby effectively suppressing the chromaticity shift in the slantedviewing direction.

It is to be noted that the “thickness d” of the second protective layer122 shown in FIG. 7 can be replaced by the “retardation” of the secondprotective layer 122 in the thickness direction, to obtain satisfactorylevels for the leakage light and the chromaticity shift in the slantedviewing direction. FIG. 28 shows the satisfactory level for thenormalized chromaticity shift obtained by the combination of theretardation of the second protective layer 122 and the retardation ofthe optical compensation layer 117. As understood from FIG. 28, thesatisfactory normalized chromaticity shift can be obtained if theoptical compensation layer has a retardation of 83 to 210 nm for thecase where the second protective layer 122 has a retardation of 0 to 18nm.

The principle of the present invention may be preferably applied to anIPS LCD device including polarizing films having a specific haze valueon the surfaces thereof, the specific haze value being obtained by asurface treatment for improvement of visibility. Such an LCD devicegenerally has significant leakage light in the slanted direction upondisplay of black. This causes a problem that the direction of theemitted light in a slanted direction is changed to the normal directiondue to the surface treatment, thereby raising brightness upon display ofblack and degrading the contrast ratio. The present invention suppressthe leakage light in the slanted direction upon display of black,thereby reducing the brightness upon display of black and thus improvingthe contrast ratio in the normal direction in the LCD device. Thepresent invention also reduces the chromaticity shift in the slanteddirection. Thus, the LCD device of the present invention may include avariety types of polarizing films, which are treated by a variety ofsurface treatments.

Since the above embodiments are described only for examples, the presentinvention is not limited to the above embodiments and variousmodifications or alterations can be easily made therefrom by thoseskilled in the art without departing from the scope of the presentinvention.

For example, the optical compensation layer 117, third protective layer123, polarization layer 12 and fourth protective layer 124 shown in FIG.2 may be layered one on another to form an optical compensation andpolarizing film.

Further, it is to be noted that the principle of the present inventioncan be applied to any mode LCD devices, so long as the LC layer has ahomogeneous orientation and the LC molecules therein are rotated in adirection parallel to the substrate surface for the change of gray scalelevels, similarly to the IPS LCD device.

1. An in-plane-switching-mode liquid crystal display (IPS LCD) devicecomprising: a liquid crystal (LC) layer having a first optical axis;first and second substrates sandwiching therebetween said LC layer, saidfirst and second substrates being disposed on a light-emitting side anda light-incident side, respectively, of said LCD device; first andsecond polarizing films sandwiching therebetween said first and secondsubstrates and said LC layer, said first and second polarizing filmshaving polarization axes extending normal to each other and beingdisposed on the light-emitting side and the light-incident side,respectively, of said LCD device, each of said first and secondpolarizing films including a first protective layer, a polarizationlayer and a second protective layer, which are disposed consecutively asviewed from the light-incident side, each of said first and secondprotective layers having a retardation in a thickness direction thereof;and an optical compensation layer disposed between said first polarizingfilm and said first substrate, wherein: said optical compensation layerhas a birefringence satisfying the relationship 0.0≦(ns−nz)/(ns−nf)≦0.5,where ns, nf and nz are a refractive index along an in-plane slow axis,a refractive index along an in-plane fast axis, and a refractive indexalong a thickness direction, respectively, of said optical compensationlayer; an angle between said in-plane slow axis of said opticalcompensation layer and said first optical axis is within ±2 degrees; andsaid optical compensation layer has an in-plane retardation N₁ (nm), andsaid second protective layer of said second polarizing film has aretardation R_(t2):$R_{t2} = {\left( {\frac{{npx}_{2} + {npy}_{2}}{2} - {npz}_{2}} \right) \times d_{2}}$in the thickness direction thereof, where npx₂, npy₂, npz₂ and d₂ are arefractive index along an in-plane slow axis, a refractive index alongan in-plane fast axis, a refractive index along an orthogonal axis and athickness, respectively, of said second protective layer of said secondpolarizing film; and said N₁ and R_(t2) satisfy therebetween thefollowing relationship:83.050−1.18R _(t2) ≦N ₁≦228.090−1.08R _(t2) in a range of 0≦=R _(t2)≦55nm.
 2. The IPS LCD device according to claim 1, wherein: saidbirefringence of said optical compensation layer satisfies therelationship 0.0≦(ns−nz)/(ns−nf)<0.2; said first protective layer ofsaid first polarizing film has a retardation R_(t3):$\left. {R_{t3} = \left( {\frac{{npx}_{3} + {npy}_{3}}{2} - {npz}_{3}} \right)} \right) \times d_{3}$in the thickness direction thereof, where npx₃, npy₃, npz₃ and d₃ are arefractive index along an in-plane slow axis, a refractive index alongan in-plane fast axis, a refractive index along a thickness directionand a thickness, respectively, of said first protective layer of saidfirst polarizing film; and said R_(t2) and R_(t3) satisfy therebetweenthe following relationship:48.3−1.05R _(t2)+0.00952R _(t2) ² ≦R _(t3)≦111.0−0.529R _(t2)−0.00472R_(t2) ² in a range of 0.0≦R_(t2)<37.5; and342.0−16.9R _(t2)−0.222R _(t2) ² ≦R _(t3)111.0−0.529R _(t2)−0.00472R_(t2) ² in a range of 37.5≦Rt2≦55.0.
 3. The IPS LCD device according toclaim 1, wherein: said birefringence of said optical compensation layersatisfies the relationship 0.2≦(ns−nz)/(ns−nf)≦0.4; said firstprotective layer of said first polarizing film has a retardation R_(t3):$R_{t3} = {\left( {\frac{{npx}_{3} + {npy}_{3}}{2} - {npz}_{3}} \right) \times d_{3}}$in the thickness direction thereof, where npx₃, npy₃, npz₃ and d₃ are arefractive index along an in-plane slow axis, a refractive index alongan in-plane fast axis, a refractive index along an orthogonal axis, anda thickness, respectively, of said first protective layer of said firstpolarizing film; and said R_(t2) and R_(t3) satisfy the followingrelationship:8.75−0.957R _(t2)+0.0093R _(t2) ² ≦R _(t3)≦90.3−0.368R _(t2)−0.00832R_(t2) ² in a range of 0.0≦R_(t2)<10.0;0≦R _(t3)<90.3−0.368R _(t2)−0.00832R _(t2) ² in a range of10.0≦R_(t2)<38: and431.0−22.8R _(t2)+0.302R _(t2) ² ≦R _(t3)≦90.3−0.368R _(t2)−0.00832R_(t2) ² in a range of 38.0≦R_(t2)≦55.0.
 4. The IPS LCD device accordingto claim 1, wherein: said birefringence of said optical compensationlayer satisfies the relationship 0.4≦(ns−nz)/(ns−nf)<0.5; said firstprotective layer of said first polarizing film has a retardation R_(t3):$\left. {R_{t3} = \left( {\frac{{npx}_{3} + {npy}_{3}}{2} - {npz}_{3}} \right)} \right) \times d_{3}$in the thickness direction thereof, where npx₃, npy₃, npz₃ and d₃ are arefractive index along an in-plane slow axis, a refractive index alongan in-plane fast axis, a refractive index along a thickness directionand a thickness, respectively, of said first protective layer of saidfirst polarizing film; and said R_(t2) and R_(t3) satisfy the followingrelationship:0≦R _(t3)≦65.2−0.805R _(t2) in a range of 0.0≦R_(t2)≦55.0.
 5. The IPSLCD device according to claim 1, wherein 0≦R_(t2)≦25.
 6. The IPS LCDdevice according to claim 1, wherein:(npx ₂ −npy ₂)×d ₂<10; and(npx ₃ −npy ₃)×d ₃≦10.